JP-A-2010-279158 discloses a circuit protection device that detects a current value of electric current supplied to a load (load current value) with current detection means and detects an overcurrent based on the detected load current value. In the circuit protection device, the detected load current value is compared in parallel with a plurality of comparators. Then, an addition value corresponding to the largest threshold value that is determined to exceed a threshold value by the comparators is added. When, a result of addition exceeds a predetermined determination value, the current protection device detects an overcurrent and interrupts current application to the load.
In general, it is desirable to bring a current interruption characteristic close to a deterioration characteristic of a wire to decrease a line diameter of the wire. In the circuit protection device, the comparators corresponding to the number of the threshold values of the electric current are provided. Thus, in order to bring the current interruption characteristic close to the deterioration characteristic of the wire, the number of comparators may increase, and a configuration of a circuit may be complicated. An on-off state of the current application to the load is controlled with a semiconductor switch. However, a current characteristic may change with a temperature of the semiconductor switch. The above-described circuit protection device cannot perform a temperature correction in accordance with a change in the current characteristic. Thus, the above-described circuit protection device cannot detect an overcurrent with a high accuracy.
JP-A-2009-142146 discloses a power supply device that calculates a square current value of a detection value proportional to an electric current supplied to a semiconductor switch coupled with an electric wire, compares a temperature increase corresponding value obtained from the square current value and an abnormality determination value, and turns off the semiconductor switch when the temperature increase corresponding value is greater than the abnormality determination value.
In the power supply device, a transitional thermal change of a heat flow rate corresponding to a thermal change in the semiconductor switch can be expressed by a logical expression, and the logical expression can be calculated by digital computing.
However, in the power supply device, in order to actualize the thermal change in accordance with the predetermined logical expression with a digital circuit, the digital circuit and a peripheral circuit are required. Thus, a circuit for protecting the semiconductor switch may be complicated.